Harpy eagles have the most massive legs and feet of any living eagle. The rear talon of a large female commonly measures between eight and twelve centimetres along the curve, and the tarsus can be as thick as a human wrist. Females are substantially heavier than males, often in the range of six to nine kilograms against four to five.
The design suits ambush in cluttered forest. Rather than stooping from height like a peregrine, a harpy eagle moves short distances between perches and snatches sloths, howler monkeys and capuchins directly from branches, killing largely by the puncture and crushing force of the grip.
What is often overstated is speed. Reported flight speeds for harpy eagles are modest, and much of the popular writing about them borrows numbers from unrelated falcons. Breeding output is also low: a pair typically raises one young every two to three years, which is why forest clearance hurts the species disproportionately.